Osbourne definitely has the looks for it, from those pictures I would be scared of her being Mama Morton. She isn't a "matron" though, too young for that but actually if they had cast a young Roxie and Velma (a la Ashley Simpson), this could work better. Either way, it's not as bad as I thought it would be, I'm really loving the publicity shots.
Also, keep in mind that CHICAGO is a very stylized sort of metaphorical production, it's not like the original run when they were a bit more literal (in terms of costumes and sets) so that's why they can cast younger people in the roles and be less strict about it.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"